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1!                           MONOPHYOBONT DENTITION                        53
which arise on the outer side of the fold of epithelium and some
way above its lower termination. These ultimately acquire a
bell-like form, and are as it were moulded on to a thickened con-
centration of the dermis beneath; they then become separate
from the downgrowth of the epithelium whence they have arisen.
Finally, each of the eight germs becomes one of the milk teeth of
the animal The lower end of the sheet of invaginated epi-
thelium, the common enamel germ, is the seat of the formation
of the second set of teeth, of which, however, in the animal under
consideration, there are only two in each jaw. But corresponding
to each of the enamel germs of the milk dentition, with the
exception of the first two molars, there is a slight thickening of
the end of the common enamel germ, which at a certain stage is
indistinguishable from the thickening which will become one of
the permanent teeth. We have thus the diphyodont arrange-
ment. But this does not exhaust the series of rudimentary teeth,
though no more come to maturity than those whose development
has already been touched upon. In the upper jaw a small out-
growth of the common enamel germ arises above and to the
outer side of the enamel germ of the third milk incisor; this does
not develop any further, but its resemblance to the commencing
germ of a tooth seems to indicate that it is the remnant of a
tooth series antecedent to the milk series. Furthermore, there
are indications in the fourth premolar of a fourth series of teeth
posterior in appearance to the permanent dentition. We arrive
therefore at the important conclusion that although here as
elsewhere there are only two sets of calcified teeth ever developed,
there are feeble though unmistakable remains of two other series,
one antecedent to and the other posterior to the diphyodont
dentition. The gap therefore which separates the mammalian
dentition from that of reptiles is less than has hitherto appeared.
Dr. Leche also carefully studied the tooth development of Iguana ;
he found that in this lizard there are four series of teeth which
come to maturity, and a rudimentary series antecedent to these
which never produces fully formed teeth.
In a few mammals there is a kind of dentition known as the
monophyodont, in which only one series of teeth reaches maturity j
where in fact there is 110 replacement of a milk series by a per-
manent dentition. Of the monophyodoiit dentition Whales form
an example. The Marsupials are very nearly an instance of the